Use of magneto-resistive sensors for borehole logging

ABSTRACT

A downhole caliper instrument utilizing a linear or arcuate magneto resistive sensor to determine the position of one or more caliper arms which extend from a carrier tool to touch the surface of a borehole wall. A processor and firmware are included to calculate and plot the borehole radius. A plurality of magneto resistive sensors are provided to calculate the sine, cosine and tangent for accurate motion of each independent arm. The magneto resistive sensors sense the position a precise magnetic ruler linked to the caliper arm. The position of the caliper arm determines the radius of the borehole. Calculation of Radius/Diameter is preferably performed in the computer on surface. The downhole controller&#39;s firmware controls the multiplexer switching, A/D conversion and other functions.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation in part of U.S. patientapplication Ser. No. 09/703,766 entitled “Use of Magneto-ResistiveSensors for Borehole Logging” filed on Nov. 1, 2000 by Helmut Lechen.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to the use of magneto resistivesensors for calculating the diameter of a borehole.

[0004] 1 . Back ground of the Related Art

[0005] Down hole distance and bore hole radius measurements havepreviously been performed with instruments using potentiometers, linearvoltage transformer (LVT) sensors or linear voltage differentialtransformer (LVDT) sensors. Potentiometers are resistors with anattached position indicating slider. The slider contacts the resistor atany point between the potentiometer ends. The mechanical position of theslider determines the resistance between the slide and the ends of thepotentiometer. A position measurement is thereby possible, by measuringeither the resistance or the voltage between the slider and one of thepotentiometer ends.

[0006] In down hole logging instruments, potentiometers are typicallyplaced inside of an oil-filled container, so that the mechanicalmovement of the slider is not influenced by the bore hole pressure. Theoil-filled containers are usually connected to a main pressure housingvia a pressure feed through connector and pressure resistant wires.Typically, there is hysteresis between the values measured moving in onedirection as opposed to values measured while moving in the otherdirection. New potentiometer devices exhibit small hysteresis effectsbut as the devices age, the hysteresis effects becomes more severe aspotentiometers begin to wear. Typically, the achievable resolutiondepends upon the design of the potentiometer. If a wire-woundpotentiometer is used (which is usually the case), the resistancebetween two windings determines the resolution of the wire woundpotentiometer.

[0007] LVT/LVDT sensors are also popularly used to measure distancesdownhole. The operating principle behind LVT and LVDT sensors is basedon a transformer having a movable ferromagnetic core. The physicalposition of the core inside of the surrounding windings alters thecoupling between the primary and secondary coils of the transformer.While keeping the primary supply voltage constant, the secondary voltagechanges in proportion to the position of the positional ferromagneticcore. The LVT sensor uses a single primary coil and one secondary coil.One side of the primary and secondary coils are connected, so that onlythree wires are necessary to connect an LVT device. The more accurateLVDT device uses two secondary coils. In the LVDT the difference betweenthe two secondary coil voltages is divided by the sum of the twosecondary coil voltages, thereby compensating for voltage changes due tounstable supply voltages.

[0008] Unlike potentiometer position sensors, LVT and LVDT sensors donot exhibit hysteresis or wear. LVT/LVDT sensors also experience lessresolution limitations. Thus, LVT/LVDT sensors are an improvement overpotentiometer sensors. The LVT/LVDT measurement, however, is anAC-measurement, thus, the LVT/LVDT sensor signal has to be rectified bysome means (hardware, software) for measurement. If larger distances areto be sensed, LVT/LVDT sensors become very large and very expensive.High pressure/temperature versions are possible but they are bulky andexpensive. Thus, there is a need for a downhole position indicationdevice that is accurate, durable, compact and not sensitive to downtemperature and pressure.

SUMMARY OF THE INVENTION

[0009] The present invention provides a magneto restrictive positionindication device for measuring the radius of a borehole. The presentinvention does not require any exposed wiring when used in down holelogging equipment. The present invention is more reliable than knownmeasurement systems because of the absence of hysteresis and because thepresent invention is less sensitive to downhole pressure and temperaturethan known prior systems. The present invention is also easier tomaintain. Additionally, the present invention's measurement accuracy islimited only by the mechanical and electronic components comprising thestructure, rather than by the sensor system itself. A measurementaccuracy of 50 μm (0.002 inch) is easily achievable with the presentinvention. The resolution of the present invention typically is 0.002inch. The sensor of the present invention also requires less space thanall other known measurement systems.

[0010] The distance measured by the preferred embodiment is preferably 5mm (just less that ¼ inch). By combining two of these measurements,distances of up to about 1.5 inches can be reliably measured. Combiningthree measurements enables distances of up to 10 inches to be measuredwith the same accuracy and reliability over the full pressure andtemperature range downhole.

[0011] The structure of the present invention provides a unique signalamplitude between 0° and 180°. In order to obtain unique informationover the full 360° range, a second sensor (bridge) is provided, which ismechanically shifted or displaced by exactly one quarter of the interpole distance, that is, the second sensor is shifted by 90°. The outputsignal of the second sensor also resembles a sine wave, having anamplitude waveform shifted by 90° compared to the signal from the firstsensor, the cosine. The phase relationship between the outputs of thefirst sensor and the second sensor in both embodiments is depicted inFIG. 4.

[0012] Preferably, linear magnetic rulers with precisely defined interpole distances of 0.125 inch to 0.25 inch are provided in order to usethe preferred sensors which precisely measure magnetic pole position.Magnetic poles are precisely magnetized or “written” on the surface of amagnetic material. Accurately positioned magnetizing devices, which arewell known in the art, are used for the production of such preciselinear magnets or so-called “magnetic rulers.”

[0013] The tangent or ratio of the sine and cosine generated by thefirst and second sensors are calculated from these two sensor signals.Considering the magnetic pole quadrants for the tangent, thisarrangement enables the present invention to determine the exact sensedposition of a magnetic ruler over the entire range from 0° to 360° . Theresolution and accuracy of the structure of the present inventiondepends upon the comprising auxiliary hardware, including the resolutionof the analog to digital (A/D) converter. Calculating the tangentrelationship provides the additional advantage of automatic compensationfor temperature drifts and distance changes encountered by magnet/sensorassembly of the present invention.

[0014] In a preferred embodiment, the arms of the preferred caliperinstrument move a sliding or rotating linkage connected to a magneticruler and pivoting end of each caliper arm. The caliper arm movement andassociated magnetic ruler are sensed by a magneto resistive sensor,which is contained in a pressure tight housing of the instrument. Thepresent invention provides a pressure tight sensor housing using anO-ring sealed connection to the central housing (which for simplicity isnot shown on the drawing). Thus, the measurement instrument does notrequire any electrical connections exposed to the bore hole fluid.

[0015] While one sensor can accurately measure the distance between twomagnetic poles, two or three measurements are combined for determiningthe absolute position of the magnetic ruler over large position ranges(5″ or more). In a preferred embodiment, the magnetic ruler is in formof a disk or half disk. The magneto resistive sensors determine theangular rotation of the turning disc shaped magnetic ruler as the outerend of the caliper arm turns. The circular magnetic ruler is attached tothe pivoting end of the pivotally attached caliper arm to determine theradius of the borehole. The borehole diameter and slope are calculatedfrom the radius measurements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a side view of a preferred embodiment of the presentinvention;

[0017]FIG. 2 is a cross section of a preferred embodiment of the presentinvention;

[0018]FIG. 3 is an illustration of the measurement geometry of thepresent invention;

[0019]FIG. 4 is an illustration of the phase relationship of the sensorsof the present invention;

[0020]FIGS. 5a and 5 b are schematic illustrations of the electronics ofa preferred embodiment of the present invention;

[0021]FIG. 6 is a top view of a preferred embodiment deployed in aborehole; and

[0022]FIG. 7 is an illustration of an alternative embodiment of thepresent invention.

[0023]FIG. 8 is an illustration of the alternative caliper arm;

[0024]FIG. 9 is a more detailed drawing of the alternative caliper armand linkage;

[0025]FIG. 10 is a schematic diagram showing a CPU and caliperinterface; and

[0026]FIG. 11 is an illustration of the field direction across magneticpoles as it changes direction between 0° and 180° between two poles.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0027] Turning now to FIG. 1, the present invention provides a six-armcaliper instrument comprising six radial extensive, equally spacedcaliper arms, each of which independently enables an independent radiusmeasurement of the borehole in which the caliper instrument is deployed.The six-arm caliper instrument 10 provides six independent arms 11, eachof which comprises two spring-loaded telescopically extensible sections14 and 16, as illustrated in FIG. 1. The caliper instrument of thepresent invention enables logging of independent borehole radiusmeasurements associated with each six caliper arms, while the caliperinstrument traverses the bore hole 22. The tool may alternately have 1-6or more caliper arms. Borehole radius measurements are taken and loggedduring ingress and egress of the caliper instrument in the borehole, andwhile the caliper instrument is descending or ascending the borehole.The preferred caliper arm 11 comprises two spring-loaded telescopicsections 14 and 16, which enable the caliper arm sections to extend orcompress to conform to the diameter of the borehole. A pad 18 isattached between the upper and lower telescopic arm sections 14 and 16of each caliper arm 11. The pad 18 engages the borehole wall 20 tomeasure caliper arm deflection angles to calculate the radius of thebore hole, as shown in FIG. 6. The caliper arms 11 change length and theangle of attachment to the instrument body of upper arm section 12 andlower arm section 13, as the instrument 10 traverses the bore hole 20and the radius 26 of the bore hole 20 changes.

[0028] Turning now to FIG. 2, in order to measure the movement of arm 11upper section 12 and lower section 13, magneto resistive angle sensorsare placed at the pivotally attached ends of upper caliper arm section12 and lower caliper arm section 13. FIG. 2 illustrates a cross sectionof the preferred angle sensing mechanism of the present invention. Theangle change of each telescopic arm section is sensed by magnetoresistive angle sensor 24. Sensor 24 is located at the end of the armsection where each arm section is pivotally attached via pin 28 toinstrument body 22. The pivotally attached caliper arm end contains asmall half-ring magnetic ruler 26, which is precisely marked withmagnetic poles and calibrated for precise measurement of the angle 32between the arm section 14 and instrument body 22.

[0029] The magnetic sensor 24 is contained in a pressure housing 22,which forms the body of the instrument. A sensor housing 25, whichcontains the sensor 24, is coupled to the mandrel via an O-ring sealedmechanical coupling. Each caliper arm 11 comprises two telescopicsections. Each telescopic section comprises an inner section 16 and anouter section 14. The inner arm section is smaller in diameter enablingthe inner arm section to telescopically slide into the larger outer armsection. A spring 34 inside of the larger telescopic arm section engagesand urges the smaller inner telescopic arm section outward to itsoutmost position. A kick off spring 36 urges the telescopic arm sectionto swing out from the instrument body 22. Both the telescopic arm spring34 and the kick off spring 36 are limited in expansion by the pad 18 ofthe caliper arm 11 contacting the surface of the bore hole wall 20,thereby defining the radius of the bore hole.

[0030] In the preferred embodiment of FIG. 2, the sensors are able tomeasure one pole-distance of the half-ring magnet. The magnets aremagnetized for 10 poles on the circumference, i.e. one pole distance isequivalent to 36°. The resolution for the measurement is determined bythe resolution of the A/D converter, the quality of the electronics andthe instrument firmware. In the case of the preferred embodiment, theresolution is set to 0.036° by the firmware.

[0031] Turning now to FIG. 3, the radius 26 of the bore hole 20 isdetermined by the triangle formed by the two pivoting telescopic caliperarm sections 12 and 13 and the constant base length 15 formed by thedistance between each arm section hinge 28, which attaches the pivotingarm sections to the instrument body 22. The angles 30 and 32, betweenthe base and each of the arms, is measured, as shown in FIG. 3. Theseangle measurements fully determine the triangle formed by the caliperarm sections and the instrument body. Thus, the height of the triangle,which is equivalent to the radius 26 of the borehole can be calculated.In a preferred embodiment, the radius measuring range is 1{fraction(13/16)} inch to 13 inches.

[0032] The instrument provides user selectable data sets that enable theuser to select which of the available data to send to the surface. Theuser can also select a test data pattern to send to the surface fortesting communications. In an alternative embodiment, the data is storedin the tool downhole in memory on board the electronics board for laterretrieval. Auxiliary signals comprising supply voltage and instrumenthousing temperature are also sent to the surface or alternatively storedin the tool. A preferred data set provides radii calculated in theinstrument and enables quick-look tests wherein radii are determined byreference to a look up table indexed by the caliper arm anglemeasurements.

[0033] The spring-loaded telescopic caliper arm sections enable theupper arm to collapse and reduce in length as the instrument descendsinto the borehole. Similarly, while logging during ascension of theinstrument, the lower caliper arm section collapses as necessary. Theindependent telescopic nature of the caliper arms enables each arm to beclosed and fully retracted, even if one of the other arms is stuck inits most extended position.

[0034] As shown in FIG. 3, angles 30 and 32 between the telescopic upper12 and lower 13 caliper arm sections and the instrument body 22 aresensed by magneto resistive sensors. From these two angles and the knowndistance 15 between the respective arm section hinges, the radius 26 canbe calculated for each arm. FIG. 3 shows two possible arm positions withtwo different radii 26 and 27, two different sets of lower arm sectionangles 30 and 31 and upper arm section angles 32 and 33.

[0035] The relationship between the measured angles 30 and 32 of eachtelescopic arm section and the radius R 26 is given by the geometry ofthe instrument as shown in FIG. 3. In a preferred embodiment, the fullyclosed instrument has a diameter of 3.625″ or 92.075 mm, which isequivalent to a radius R of 1.8125″or 46.0375 mm. Using the actualinstrument dimensions, the relation between the radius of the bore hole,R and the measured angles 30 and 32 is as follows:${R({mm})} = {46.0375 + {\frac{{{tg}({Lower})} \cdot {{tg}({Upper})}}{{{tg}({Lower})} + {{tg}({Upper})}}*1107}}$

[0036] All dimensions for this equation are in mm. The conversion toinches is achieved by dividing the result by 2.54. The preferredfirmware provided in the instrument of the present invention enables anaccurate angle measurement and calculation of radius or diameter. Duringradius logging, in a preferred embodiment, a surface computer system isused to calculate the radii from these accurately measured angles, usingthe above equation.

[0037] Two sensors are used per caliper arm, one to sense the angle ofthe telescopic upper arm section, and the other one to sense the angleof the lower telescopic arm section. In a preferred embodiment, theresolution of the measurement system is 0.036°, which is provided in1000 increments for an angle measurement range of 0 to 36°. At thelargest radius measured (13 inches), the resolution of 0.036° provide aradius resolution of 0.44 mm (0.017 inch). Near the closed position(1{fraction (13/16)} inch) a radius resolution of 0.35 mm (0.014 inch)is provided.

[0038] The springs 34 in the telescopic caliper arm sections do notprovide a significant opening force when the arms are fully inside themandrel. Therefore, leaf kick out springs 36 mounted at the ends of thearms are provided to help force the arms from the body for the first fewinches.

[0039] In the preferred data acquisition mode, the caliper instrument ofthe present invention reads all caliper telescopic arm section sensors,calculates the various distances and places this data in a buffer. Thesensor data is then transmitted to the surface, stored or processedimmediately down hole in the instrument. As soon as a request for datais received, the collected data is sent and a new data collection cycleinitiated.

[0040] In a preferred embodiment, the following data sets can berequested by the user:

[0041] Data Set 1)

[0042] Six Radii, Temperature, Voltage

[0043] Data Set 2)

[0044] Main logging data set. For each arm the upper and the lower angleare transmitted. Additionally, Temperature, Voltage, A/D offset aresent.

[0045] Data Set 3)

[0046] Maintenance data set. For each sensor, the A/D reading is given.There are 24 sensors in the instrument, 2 per arm-half.

[0047] Data Set 4)

[0048] Communication test pattern.

[0049] The caliper instrument of the present invention provides precise,high resolution information regarding the radial position of the caliperarms and proximity of the tool string to the bore hole wall. Thisinformation determines the distance from the tool string to the boreholewall. The borehole radius also infers the proximity of adjacentformations to the other logging instruments deployed in the borehole inthe same tool string with the caliper instrument of the presentinvention. For example, a nuclear magnetic resonance (NMR) tool designto deploy near or against the borehole wall may require correction formeasurements taken when the NMR tool is not adjacent or touching thebore hole wall. The caliper instrument can be combined with such an NMRtool or any other down hole instrument to measure the distance of thetool string to the bore hole wall. Thus, the present invention can bedeployed on the same tool string with a formation proximity sensitiveinstrument to determine proximity and accommodate proximity correctionsfor such a tool.

[0050] Tool string orientation (north, south, east, west) is also sensedby the present invention along with the orientation of the NMR tool andother associated downhole tools, to facilitate proximity correction ofdirection sensitive instruments and measurements. The caliper instrumentof the present invention does not centralize the downhole tool string.The present invention only senses the radial position of the tool stringin the borehole by obtaining the radii measurements on the six caliperarms. Using the information provided by the caliper arms along withorientation data, the present invention enables accurate bore holecorrections to be applied for the formation proximity and orientationsensitive instruments deployed on the tool string.

[0051] The actual position of the sensing arm can move above or below acenter position, so that the measuring point may be shifted duringlogging. The amount of movement is preferably less than ±1.75 inch andcan actually be calculated from the data sent by the instrument, so thatprecise radius curve depth corrections are possible.

[0052] The present invention enables well bore radii or diameterplotting as well as an average bore hole diameter calculation.Measurement errors are minimized by placing caliper calibration rings onthe instrument and performing a two-point calibration. The presentinvention is preferably calibrated as follows. The preferred caliperinstrument is connected to a cable head via a common remote instrument.A small diameter calibration ring, e.g., 8-inch is applied to thecaliper arms and an angle reading a recorded. A small diametercalibration ring, e.g., 12-inch is the applied to the caliper arms andanother reading is recorded. The rings simulate borehole radiusmeasurements. The calibration readings are used to adjust for anglemeasurements and radius or diameter calculations made during actualoperations in the bore hole. Typically, the caliper instruments arecalibrated on site and then verified inside the surface casing. Forverification before logging, the tool is stopped inside casing, beforelowering the instrument into an open hole. A verification routine(before log verification) is performed and a reading recorded. To verifythe instrument after logging, after retrieving the caliper instrumentback into the casing, the operator stops the cable hoist and selects averification routine, records a reading and enters it for a secondverification point.

[0053] In a preferred embodiment, to begin operations, the preferredcaliper instrument is connected to a cable head via a common remoteinterface. The instrument is powered by connecting the instrument to a180 VDC power source. The instrument is then calibrated per thecalibration instructions as shown above. The power is then removed andthe caliper instrument connected anywhere within the downhole toolstring. After calibration, the instrument is now ready for bore holeradius logging.

[0054] Turning now to FIG. 5, the electronics 40 of the preferredinstrument are contained in the upper pressure housing, as shown in FIG.5. The electronics comprise a power supply 42, a CPU board 44 and acaliper board 46. The power supply consists of a transformer 47,rectifier 48, three voltage regulators 49 and capacitors 50. The powersupply provides preferred power for the caliper boards and the preferredCPU board. This CPU board contains an instrument bus interface, memoryand a serial I/O port. The preferred caliper instrument utilizes thisI/O port. The signal-out lines are used to control the caliper boardsand the signal-in lines are used to read the information from thecaliper sensors. Together with a ground wire, 5 wires are used betweenthe CPU board and the caliper board. Any suitable CPU board such as anIntel Pentium processor may be substituted for the preferred CPU board.The caliper board contains multiplexers, sample and hold circuits, shiftregisters, analog to digital converters and tri-state buffers for dataacquisition and transmission.

[0055] Preferably, an 8-bit shift register is used to control thefunctions of the caliper board. One control sequence consists of 8 bits.After the CPU board sends the respective data sequence to this register,the caliper board executes the 8-bit command.

[0056] The 16-bit A/D converter converts the following signals in thefollowing sequence:

[0057] Signal Sine of Lower Sensor of Arm 1;

[0058] Signal Sine of Upper Sensor of Arm 1;

[0059] Signal Cosine of Lower Sensor of Arm1;

[0060] Signal Cosine of Upper Sensor of Arm 1;

[0061] The same sequences are repeated for Arms 2 to 6; and

[0062] The seventh sequence collects auxiliary data (voltages, offset,temperature).

[0063] Signals are first selected, then the sample and hold circuits areswitched to hold and all signals for one caliper arm pair (lower andupper sections) are then sequentially converted. The sample and holdoutputs are switched to a multiplexer for output. The output of themultiplexer is connected to the A/D converter via an operationalamplifier.

[0064] The A/D converter outputs are clocked into the signal input lineof the CPU board. At the end of one complete cycle, 24 sensor signals(12 sine and 12 cosine) are available on the CPU board. The sensoroutputs are temperature-sensitive. For that reason, and for greaterprecision, the ratio of the sine and the cosine signals is calculated(tangent) and used for further calculations. Most of the temperature andother error producing influences cancel out in the tangent calculations.

[0065] The direction of the magnetic field of a circular magnet, wherethe poles are side by side, with alternating polarity, changes frombetween 0 to 180 degrees between two poles. The magnetic field isperpendicular to the surface plane of the magnet at the poles andparallel to the surface plane of the magnet in the middle between thetwo poles. The magneto resistive sensors of the present invention areonly sensitive to alignment with the magnetic field, and not sensitiveto the absolute direction of the magnetic field. Hence, the resistanceof the first half bridge sensor reaches its minimum directly over one ofthe poles. The maximum sensor resistance is reached when the magneticfield is perpendicular to the sensor axis, that is, in the middlebetween the two poles 52 (cosine). The second half bridge sensor ismechanically rotated by 90°, having its output shifted by 90° comparedto the first one 54 (sine). FIG. 4 illustrates the output voltages ofthe first sensor 52 and the second sensor 54, as the pair is movedacross the distance.

[0066] The output voltage of a single sensor (half bridge) does notuniquely determine the position of the sensor between the two magneticpoles, as there are two positions possible for every value of the curve,except for the highest and lowest voltages. Also, if only one sensorwere used to determine the position, the output voltage would have to bevery stable. If the ratio of the two sensor voltages is taken (tangent),then the position can be exactly determined and the influences ofsensitivity changes due to changing temperatures and distance changesbetween sensor and magnet cancel out in the tangent calculation. For thedetermination of the angle it is, of course, necessary to consider thequadrants.

[0067] External magnetic fields are added to the magnetic field of themagnet. This means, the magnetic field adds at one place and subtractsat another place, where the field of the magnet is oppositely directed.This would lead to incorrect calculations of the position, as the sineand cosine output are affected in the same direction. In order tocompensate for this effect, a second series of sensors is placed exactlyone pole distance apart from the first. From FIG. 4 above, it can beseen that the output signal of the sensor is the same above the Northand the South pole. The external field, however would add to one andsubtract from the other. In other words, a second sensor set, one poledistance apart from the first, provides the same position information asthe first, the influence of the external magnetic field, however, is inthe opposite direction. The signals of the two sensor sets added, andthe total number of signals is not increased. This compensation works,as long as the external field experienced by the two sensor sets issubstantially the same.

[0068] The maximum magnetic induction of the ruler magnets is about 0.1T (1000 G). Values substantially below this value do not influence themeasurement. Magneto-resistive sensors are preferably used for anglemeasurements because they are insensitive to the harsh down holeenvironment and can be mounted in non-magnetic pressure housings. Highprecision accuracy is achieved with these sensors and essentiallylimited only by the electronic components used to convert the voltagesto digital signals.

[0069] Turning now to FIG. 7, an alternative embodiment of the presentinvention is shown. The alternative embodiment uses rigid non-telescopicarms 111 that slidably attach to the tool body 22. For logging duringdownward progression in the borehole provides a full stop for the arms111 on the lower side and free movement of the arms on the upper side.For logging upward in the borehole, the arm stop is on the upper sideand the lower arms are freely movable. The preferred design provides twosliding sections which are hinged to the arms, as shown in FIG. 8. Themovement of both, the upper sliding section and the lower slidingsection is sensed and measured. From this measurement, the actualborehole radius and diameter are calculated. In this configuration,there is very small movement for an instrument in the almost closedpostion, thus, the angular movement is additionally sensed. The twomovements are mechanically added together and are measured using thesame position sensor.

[0070] Referring now to FIG. 9, the angle change of the arm 111 istransferred into an axial movement by a small linkage. Linkage arm 90 isfixed to the caliper arm 111, linkage arm 92 is fixed to a magneticruler 94 and linkage arm 96 connects the two together. The magneticruler 94 slides inside the sliding section 98, this way moving togetherwith the sliding section plus moving additionally if the caliper armangle changes. A small spring holds the magnetic ruler under tension, sothat mechanical play is substantially removed. The sliding section 98movement is sensed by the sensor 102 which is located below the magneticruler 94.

[0071] The relation between the movement m and the borehole radius R isgiven by the geometry of the instrument—which is constant. For thecalculations, the ‘net radius’ r is used. The value for r is 0 if theinstrument is fully closed. The fully closed instrument has a diameterof 3.625″ or 92.075 mm, which is equivalent to a radius R of 1.8125″ or46.0375 mm.

[0072] Using the actual instrument dimensions, the movement m can becalculated as follows:

r=R−Instrument Body Radius

[0073] and $m = {{\frac{L2}{L1}r} + {L1} - \sqrt{{L1}^{2}} - r^{2}}$

[0074] All dimensions can be in mm or inches. The conversion to inchesfrom mm is achieved by dividing the numbers by 25.4. The length m is theaverage of the movements of the two sliding sections. Solving the sameequation for r, the radius can be calculated from the movement.

[0075] A schematic illustration of the CPU board and caliper board isdepicted in FIG. 10. The caliper board contains multiplexers 125, S/Hcircuits 127, shift register, A/D converter 129 and tri-state buffers.The 8-bit shift register is used to control the functions of the board.As two boards are used, one control sequence consists of 16 bits. Thetwo shift registers are connected in series, so that the signal for thesecond register is shifted through the first. In any case, after the CPUboard sent the respective data sequence to these registers, the caliperboard behaves as desired.

[0076] The 16 bit A/D converter converts the following signals in thefollowing sequence:

[0077] Signal Sine of Lower Sensor A of Arm 1;

[0078] Signal Sine of Upper Sensor A of Arm 1;

[0079] Signal Sine of Lower Sensor B of Arm 1;

[0080] Signal Sine of Upper Sensor B of Arm 1;

[0081] Signal Sine of Lower Sensor C of Arm 1;

[0082] Signal Sine of Upper Sensor C of Arm 1;

[0083] Signal Cosine of Lower Sensor A of Arm 1;

[0084] Signal Cosine of Upper Sensor A of Arm 1;

[0085] Signal Cosine of Lower Sensor B of Arm 1;

[0086] Signal Cosine of Upper Sensor B of Arm 1;

[0087] Signal Cosine of Lower Sensor C of Arm 1;

[0088] Signal Cosine of Upper Sensor C of Arm 1; and

[0089] The same sequences are repeated for Arms 2 to 6.

[0090] Signals are first selected, then the S/H circuits 127 areswitched to hold and all signals for one arm pair (lower and upper) aresequentially converted. The S/H outputs feed a multiplexer 125. Theoutput of the multiplexer is connected to the A/D converter 129 via anoperational amplifier 130. The A/D outputs are clocked into the signalinput line of the CPU board.

[0091] The advantage of not using any exposed wiring is provided byusing magnetic sensors. Two sensor sections are used per segment, one tosense the movement of the upper sliding section, the other to sense themovement of the lower sliding section (plus arm angle change asexplained above). Due to the small rates of movement near the closedinstrument position, very precise measurements are made. The resolutionof the preferred system is 5 μm (0.0002″), the accuracy is in the rangeof 25 μm (0.001″).

[0092] Three linear magnets (“magnetic ruler”) with pole distances of 5mm (A), 6 mm (B) and 6.25 mm (C) are used. Three magnetic sensor pairsare used to sense the position of each of the three magnets of theruler. All sensors used for sensing one sliding section position arecontained inside one pressure housing. The pressure housing has onecentral O-ring seat which is used to feed the wires through into thecenter tube of the instrument.

[0093] The 6-Arm Caliper is spring powered. Each arm is connected to aspring 110 which is hooked to the center tube. The arm opening forceprovided by these springs becomes very small for small diameters. Leafsprings mounted in the center help providing the force for the first fewinches.

[0094] The CPU board contains a bus interface and a serial I/O port. Thegate signal and the clock signal are looped back. The signal-out linesare used to control the caliper boards and the signal-in lines are usedto read the information from the caliper sensors. Together with ground,5 wires are required between CPU board and caliper board. At the end ofone complete cycle, 24 sensor signals (12 sine and 12 cosine) areavailable on the CPU board.

[0095] The sensor outputs are temperature-sensitive. For that reason,the ratio of the sine and the cosine signal is calculated (tangent) andused for further calculations. This way, most of the temperatureinfluences cancel out.

[0096] The preferred embodiment described above and shown in the figuresis not intended to limit the scope of the invention, which is defined bythe following claims.

[0097] In an alternative embodiment shown in FIGS. 7, 8 and 9,magneto-resistive sensors are used for detecting the direction ofmagnetic fields. The sensors change their resistance depending on thealignment of the magnetic field direction compared to the sensor axis.The transfer function is:

R=R ₀ +ΔR•cos(2•Φ)

[0098] where

[0099] R is the measured resistance

[0100] R₀ field independent resistance

[0101] ΔR field dependent resistance

[0102] Φ magnetic field angle

[0103] The field direction 140 across magnetic poles 150 changes from 0°to 180° between two poles 150, This is illustrated in FIG. 11. Due tothe transfer function as given above, the value of R is the same at the0° position and the 180° position—cos(2*0) is the same as cos(2* 1 80).

[0104] In a preferred embodiment, two sensor bridges are contained on achip, the two bridges are designed to be sensitive to two differentfield directions. The first sensor bridge has its maximum sensitivity at0° fields, the second sensor bridge has maximum sensitivity at fieldswhich are 90° off the first field. In an alternative embodiment, toachieve this 90° relative response, the second sensor bridge is actuallyrotated by 45° compared to the first. The outputs of the twohalf-bridges are fed into amplifiers; at the amplifier outputs the twosignals are available.

[0105] As for linear magnets, the direction of the magnetic field of acircular magnet where the poles are side by side, with alternatingpolarity, changes from 0° to 180° degrees between two poles. Themagnetic field is perpendicular to the surface of the magnet at thepoles and parallel to the surface of the magnet in the middle betweentwo poles.

[0106] Magneto-resistive sensors are only sensitive to the alignmentwith the magnetic field, not to its absolute direction. Hence, theresistance of the first sensor (half bridge) reaches its minimumdirectly over one of the poles; the maximum of the sensor resistance isreached when the magnetic field is perpendicular to the sensor axis-inthe middle between the two poles (cosine). The second sensor (halfbridge) is mechanically rotated by 45°; its output is shifted by 90°compared to the first one (sine).

[0107] The above examples are intended to be exemplary only and notintended to limit the scope of the invention.

What is claimed is:
 1. A caliper instrument for calculating the radiiand diameter of a bore hole comprising: an instrument body having atleast one arm pivotably attached to the body for contacting the surfaceof the bore hole wall; a magnetic ruler linked to the arm; and a magnetoresistive sensor for sensing the position of the magnetic ruler fordetermining the position of the arm.
 2. The caliper instrument of claim1 further comprising: a processor for calculating the radius of the borehole from the position of the arm.
 3. The caliper instrument of claim 2wherein the magnetic ruler comprises a rectilinear element linked to thearm for determining the position of the arm.
 4. The caliper instrumentof claim 2 wherein the magnetic ruler comprises a curved element linkedto the arm for determining the position of the arm.
 5. The caliperinstrument of claim 2 wherein the sensor comprises a first and secondsensor each located to produce a first signal 90° from a second signalfor calculating the tangent of the sensed signals.
 6. The caliperinstrument of claim 2 wherein the calculated radii are plotted asindependent radii.
 7. The caliper instrument of claim 2 wherein thecalculated radii are plotted as independent diameters.
 8. The caliperinstrument of claim 2 wherein the calculated radii are plotted as anaverage bore hole diameter.
 9. The caliper claim 5 wherein the first andsecond magneto resistive sensors are separated linearly by a distanceequal to 1.2 of the interpole distance to generate a first and secondsignal separated by 90°.
 10. The caliper claim 5 wherein the first andsecond magneto resistive sensors are rotated by 45° to generate a firstand second signal separated by 90°.
 11. A method for calculating thediameter of a bore hole comprising: for contacting the surface of thebore hole wall with an instrument body having at least one arm attachedto a body; sensing a magneto resistive sensor signal to determine theposition of a magnetic ruler for determining the position of the arm;and calculating a diameter of the borehole, from the signal .
 12. Themethod of claim 11 further comprising: calculating the diameter of thebore hole from the position of the arm.
 13. The method of claim 12wherein the step of sensing a magneto resistive sensor signal comprisessensing the position of a rectilinear element linked to the arm fordetermining the position of the arm.
 14. The method of claim 12 whereinthe step of sensing a magneto resistive sensor signal comprises sensingthe position of a curved element linked to the arm for determining theposition of the arm.
 15. The method of claim 12 wherein the step ofsensing a signal comprises a signal from a first and a second magneticpole sensor located 90° apart for calculating the tangent of the sensedsignals.
 16. The method of claim 15 further comprising plotting thecalculated radii as independent radii.
 17. The method of claim 15further comprising plotting the calculated radii as independentdiameters.
 18. The method of claim 15 wherein the calculated radii areplotted as an average bore hole diameter.
 19. The method of claim 15wherein the step of sensing a signal comprises sensing a signal from afirst and a second magneto resistive sensor separated linearly by adistance equal to one half of the interpole distance.
 20. The caliperclaim 15 wherein the step of sensing a signal comprises sensing a signalfrom a first and second magneto resistive sensor rotated by 45° relativeto each other.